Operator (Add Round Cube) doesn't work
If G. Garone's answer doesn't work and none of your operators work, then i have another possible solution. I've had this happen a few times to me already, using 2.8.

Just press F3 in the 3D view, then search for "Load Factory Settings". Click on it, and the problem should be fixed. Be sure to go through your settings again, enabling add-ons and changing whatever you need. Don't forget to save the settings afterwards!